The present invention relates to a novel process for cross-linking a halogenated polymer, particularly polyvinyl chloride (PVC) or chlorinated polyethylene, by means of a plurifunctional chemical coupling agent based on thiolate. The present invention also relates to the resins cross-linked in this way.
As is known, cross-linking is an operation intended to join together the chains of a polymer by means of chemical bonds, in order to create a macromolecular network. The consequences of cross-linking a polymer are generally those of considerably modifying its mechanical properties, such as reducing its thermoplasticity and plastic flow, particularly at high temperatures, also of reducing deformation under compressive conditions and finally of improving the resistance to abrasion and the dimensional stability.
In the following specification and claims, the expression "halogenated polymer" designates a polymer comprising ##STR3## groups, X representing a halogen atom, preferably a chlorine atom. PVC or chlorinated polyethylene may be mentioned by way of example.
Several processes are already known for cross-linking these chlorinated polymers, particularly PVC in the condensed, solid or molten state.
One of these processes known under the name of NAKAMURA and described, for example, in the Journal of Molecular Sciences, Chemistry A (12), 2, 209 (1978), consists in using a coupling agent which is a derivative of triazine, namely dibutyl amino 2, dithiol 4-6 triazine, in the presence of MgO. This coupling agent comprises two thiol functions of equivalent reactivity. Cross-linking of the polymer is thus provoked by interchain chemical reactions bringing about a considerable modification in the viscosity of this polymer, which is hardly compatible with the rheological conditions imposed by the processing operations such as extrusion, injection, but especially calendering. Although this coupling agent may be used in coating processes, it nevertheless provokes a degradation of the polymer associated with the presence of tertiary amines.
In Patents Abstracts of Japan, Vol. 7, No. 277 (C-199) [1422] of Dec. 9, 1983, it was proposed to add to these triazine derivatives hydrated alumina and magnesia carbonates. This process also brings about a degradation of the polymer during the operations of working, particularly by liberation of hydrochloric acid.
U.S. Pat. No. 4,234,705 suggests using as coupling agent a mixture of derivatives of 3,5-dimercapto 1,2,4-triazole comprising amine groups, particularly tertiary amines, and of an acid acceptor based on a metallic compound of groups II or IVa. Unfortunately, the presence of the secondary or tertiary amine functions leads to the same drawbacks as the triazine proposed by NAKAMURA. In fact, it is known that these amines are pro-degrading agents, with the result that the cross-linking effected from coupling agents comprising such functions makes use of double bonds of the halogenated polymer coming from the degradation and not from the nucleophilic substitution of the chlorine atoms.